Scandinavian Mountains over 2000 metres - James Baxter

In the winter Scandinavia is a harsh environment. Consequently there is not a rich mammal presence. Even in the surrounding valleys, where there is some protection and under the snow some food, life is a struggle for most. However, in the summer conditions are kind and most mammals, especially the herbivores, flourish.

Key to how the carnivores flourish and breed is how the small rodents, like lemmings and voles breed. About every 3-5 years there is a population explosion or lemming year. It is thought that after such a year, the plants which have sustained the small rodents and been severely gnawed produce unpalatable chemicals which hinder the rodents' digestion. These chemicals only last a few years in the plants and are then weak enough for the rodents to feast again.

In lemming years many carnivores and birds of prey maximize their breeding. Indeed, some can only breed successfully in such years. The resulting abundance of predators the following year also hinders any small rodent population explosion and the population wanes again.